1. Field of the invention
The present invention relates to a solid state relay, and more specifically to a solid state relay having a MOSFET (metal-oxide-semiconductor field effect transistor) output circuit and a thyristor discharge circuit.
2. Description of related art
One typical example of conventional solid state relays have been disclosed in U.S. Pat. No. 4,754,175. For example, FIG. 3 of this U.S. patent shows a solid state relay including a light emitting diode (abbreviated "LED" hereinafter) 2, which has its anode and its cathode connected to a pair of input terminals 3 and 4, respectively, so that when an input electric signal is applied between the input terminals 3 and 4, the LED 2 emits light. In addition, there is provided an array 5 of series-connected photodiodes 6 located to receive the light emitted from the LED 2 so as to generate a photo voltage corresponding to the received light. An anode 7 of the photodiode at one end of the photodiode array 5 (simply called an "anode of the photodiode array 5" hereinafter) is connected to an anode of a diode 17, which in turn has its cathode connected to a gate of a MOSFET 9+10, so that the photo voltage generated by the photodiode array 5 is supplied through the diode 17 to the gate of the MOSFET. A source and a drain of the MOSFET are connected to a pair of output terminals 15 and 16, respectively.
The gate of the MOSFET is also connected to an anode 19 of a thyristor 18, which has its cathode 20 connected to a back gate of the MOSFET. In addition, a negative gate 21 of the thyristor 18 is connected to the anode of the diode 17, and a positive gate 22 of the thyristor 18 is connected to a cathode 8 of the photodiode at the other end of the photodiode array 5 (simply called a "cathode of the photodiode array 5" hereinafter). The cathode of the photodiode array 5 is also connected to a cathode of another diode 30, which has its anode connected to the cathode 20 of the thyristor 18. In this construction, the thyristor 18 and the diodes 17 and 30 cooperate to constitute a discharge circuit for discharging an electric charge accumulated on the gate of the MOSFET when the LED 2 is deenergized.
This solid state relay operates as follows: When the input voltage signal is applied between the input terminals 3 and 4, the LED 2 emits the light corresponding to the applied input signal. In response to the light emitted from the LED 2, the photodiode array 5 generates a corresponding photo voltage between its anode 7 and its cathode 8. With this photo voltage, a photo current flows through the diode 17 to the gate of the MOSFET, so that an electric charge is accumulated on the gate of the MSOFET. Thus, the MOSFET is turned on, so that a conductive path is formed between the output terminals 15 and 16.
On the other hand, if the LED 2 is deenergized, the voltage between the anode 7 and the cathode 8 of the photodiode array 5 drops due to a self discharge of the photodiode array 5, thereby to bias the positive gate 22 and the negative gate 21 in a forward direction. As a result, the thyristor 18 is turned on, so that the electric charge accumulated on the gate of the MSOFET is discharged through the thyristor 18, and therefore, the MOSFET is turned off.
Thus, this solid state relay can realize a high speed turning-off operation, since the electric charge accumulated on the gate of the MSOFET is discharged through the thyristor 18. In addition, an energy loss is small within the solid state relay circuit.
FIG. 5 of the above referred U.S. Pat. No. 4,754,175 also discloses another solid state relay in which the above mentioned diodes 17 and 30 are replaced with a pair of phototransistors 50 and 51, respectively, which are located to receive the light emitted from the LED 2. This modified solid state relay has not only the same advantage as that of the first mentioned solid state relay, but also another advantage in which the characteristics of the solid state relay can be modified by adjusting the photo sensitivity of the phototransistors 50 and 51.
In both of the above mentioned solid state relays, however, since the diode 17 or the phototransistor 50 are connected between the anode 7 of the photodiode array 5 and the gate of the MOSFET, a voltage drop occurs in the diode 17 or the phototransistor 50. Namely, a part of the photo voltage is lost in the diode 17 or the phototransistor 50.
In addition, when the amount of light received by the photodiode array 5 is small, the photo voltage generated between the anode 7 and the cathode 8 of the photodiode array 5 is correspondingly low. In this case, when the MOSFET is brought from the on condition into the off condition, the self discharge of the photodiode array 5 becomes slow, and therefore, a substantial time is required until the MOSFET is turned off.
Furthermore, the self discharge of the photodiode array 5 is dependent upon an internal resistance of the photodiode array 5. Therefore, the turning-off time greatly varies dependently upon variation in the internal resistance of the photodiode array 5.